This invention relates to the use of an oxygen sensor in a control system for controlling the air-to-fuel ratio in an internal combustion engine. More particularly, this invention relates to the use of the oxygen sensor where a three way conversion catalyst is employed in the exhaust system.
It is known to use catalysts in the exhaust system of an internal combustion engine to oxidize unburned hydrocarbons and carbon monoxide into water and carbon dioxide and to reduce various nitrogen oxides into nitrogen and oxygen.
In order to minimize nitrogen oxide pollution as well as hydrocarbon and carbon monoxide pollution, a so-called three way conversion catalyst can be employed. It is particularly important that the combustion system operate within a narrow range of air-to-fuel ratios around the stoichiometric value when a three way conversion catalyst is employed. A stoichiometric ratio is an air-to-fuel ratio with just enough oxygen so that if combustion is complete all of the fuel will be completely burned to water and carbon dioxide and there will be no oxygen remaining. The operating parameters of the three way conversion catalyst are such that the percentage of hydrocarbons and carbon monoxide converted is substantially less as the air-to-fuel ratio becomes richer than stoichiometric and the percentage of nitrogen oxide converted to nitrogen and oxygen is substantially less as the air-to-fuel ratio becomes leaner than stoichiometric. In some circumstances the optimum compromise between the oxidation function and the reduction function is slightly off of stoichiometric but it is always very close to if not at stoichiometric.
It is known to obtain control of the air-to-fuel ratio by use of a control system in which an oxygen sensor in the path of the exhaust gases provides a signal indicating the level of oxygen in the exhaust. The signal is then used to bring the air-to-fuel ratio to a predetermined ratio, normally stoichiometric or close to stoichiometric. Even with a stoichiometric ratio, there will inevitably be incomplete burning so that there will be pollutants in the exhaust which must be removed by the threeway catalyst. Furthermore, in the operation of a vehicle where load and speed changes are continuous, there will inevitably be short duration variations of the air-to-fuel ratio above and below stoichiometric. As a practical matter it is not possible to control to a stoichiometric ratio at all instances of time. Thus only an average stoichiometric ratio can be achieved.
If the oxygen sensor or associated circuitry fails either because of a short or an open circuit, the oxygen sensor output will not indicate the actual exhaust conditions and the control system logic will then tend to force an air-to-fuel ratio which will be substantially removed from stoichiometric. The result will be highly inefficient engine burning and an exhaust gas condition which will result in a three way conversion catalyst virtually failing to function in either the oxidation mode or the reduction mode and perhaps causing production of currently unregulated gaseous emissions such as ammonia, hydrogen cyanide and hydrogen sulfide. The gases therefore exhausted to the atmosphere can contain a high pollutant content.
Accordingly, it is a major purpose of this invention to provide a control system for use with the catalyst which will permit the catalyst to continue to operate effectively even though the oxygen sensor or associated circuitry has failed.
Furthermore, there are conditions of operation such as sudden acceleration and sudden load changes, as when starting up a steep hill, in which the air-to-fuel ratio will initially swing substantially away from stoichiometric. Under such conditions, the control circuit operates to rapidly bring the ratio back to stoichiometric. It is important that any system to compensate for oxygen sensor failure or component failure not respond to such temporary deviations from stoichiometric as if they represent a failure condition. Accordingly, it is a further purpose of this invention to provide the above type of failure detection and compensatory system that will distinguish between normal operation deviations from stoichiometric and false signals due to component malfunction.